The present invention relates to refrigeration systems. More particularly, the present invention relates to a refrigeration system in which a screw compressor is employed and to the reduction of noise created by the operation of the screw compressor in the system.
Screw compressors, by their nature, create a relatively large amount of noise as a result of compressor vibration and in the form of gas pulse energy transmitted from the compressor to system components both upstream and downstream thereof. The beneficial use of the pulse energy in the discharge gas stream of a screw compressor has, in fact, been suggested to acoustically clean the walls of closed spaces, such as in the process industry, or to cause the mixing of two different gaseous media into a single homogenized gas. U.S. Pat. No. 4,923,374 is pertinent in that regard.
Svenska Rotor Maskinet AB, the owner of the '374 patent, has likewise recognized the need, in other applications, to damp the sound and noise of screw rotor machines and has conducted work in that regard as is evidenced by its U.S. Pat. No. 5,137,439. As is set forth in column 1, lines 12-14 of the '439 patent, the difficulty in damping noise in screw rotor machine is "mainly because both the inlet line and the outline line, and particularly the latter, conduct low-frequency noise of large amplitudes in the machine".
Svenska's solution to that problem in the '439 patent is to incorporate a so-called De Laval nozzle as a sound silencing device in the vicinity of the inlet and/or outlet port of a screw compressor. As is well known, the capacity of a screw compressor can relatively easily and advantageously modulated. The De Laval nozzle of the '439 patent includes a moveable valve body, biased by a substantially constant spring force, that functions to maintain a constant pressure drop across the nozzle to permit it to maintain the velocity of the gas flowing through it immediately beneath the speed of sound irrespective of the capacity at which the machine operates.
As is noted in the '439 patent, movement of the valve body within the nozzle must be damped to achieve only slow changes in gas flow through the nozzle in order to maintain the nozzle's effectiveness. Absent the use the sound silencing apparatus of the '439 patent, it is noted that a large volume silencer would be required in order to achieve effective sound damping, particularly with respect to the low-frequency impact (pulse) waves generated by the operation of a screw compressor. What is not recognized is that the nozzle apparatus suggested in the '439 patent disadvantageously requires the use of moving parts which are subject to breakdown and wear and which add to the manufacturing cost of the system in which a screw compressor is employed.
The need continues to exist for better, less expensive and mechanically more simple apparatus and mechanisms by which to achieve noise reduction in systems in which screw compressors are employed and, in particular, in screw compressor-based refrigeration systems used in the comfort conditioning of office buildings where the compressor component of the system is most often disposed in relatively small and enclosed equipment rooms or spaces.